PROJECT SUMMARY The goal of this study is to understand how mRNA translational control regulates the terminal differentiation of erythroid cells under both normal and pathological conditions. Erythropoiesis, the production of red blood cells, is essential to mammals. Malfunction of this cell differentiation process can cause severe anemias and is associated with a large number of human hematological disorders, including many bone marrow failure syndromes such as the myelodysplastic syndrome. Thus, characterizing the molecular mechanisms controlling erythropoiesis is of both biological and clinical significance. Previous studies have characterized many transcriptional regulatory networks controlling the erythroid cell differentiation. In addition, the transcriptomic dynamics during erythropoiesis have been extensively surveyed. How these RNA changes are ?read? and interpreted by the translational apparatus, the ribosome, in the differentiating erythroid cells to generate proper amounts of proteins, however, is still largely unknown. Recently, we uncovered widespread regulations of protein synthesis during terminal erythropoiesis by parallel RNA and ribosome profiling on primary erythroid cells at different developmental stages. Specifically, during terminal erythropoiesis, we identified hundreds of differentially translated mRNAs, and their 3' untranslated regions have significantly enriched binding motifs of several erythroid-specific RNA-binding proteins, implying translational regulatory networks. Moreover, we found novel forms of translational regulations including dynamic usage of upstream open reading frames, alternative translation terminations, and stoichiometric synthesis of multi-subunit complexes. These results strongly argue for critical roles of dynamic translational control in erythropoiesis. Interestingly, mutations in several components of the cellular translational apparatus, the ribosome, can cause several human genetic diseases with manifestations of ineffective erythropoiesis, such as the Diamond-Blackfan anemia and the 5q- syndrome. These clinical observations highlight the importance of studying translational regulations in erythroid cell differentiation. In this study, we will: a) characterize the translational regulatory networks mediated by key erythroid-specific RNA-binding proteins in normal erythropoiesis; b) determine how disease-associated ribosomal protein mutations alter mRNA translation in erythroid cells. The results from this study will not only fill an important knowledge gap in erythropoiesis, but also will provide important molecular insights into ribosomopathies and potentially identify therapeutic targets for these human diseases.